bill.uk: Hey guys, here's what I got from support. It didn't work but perhaps you will have better luck.
First, please redownload and reinstall the Downloader, just in case you're using an old version
If that doesn't help, delete the contents of your user account's GOG.com folder - its location depends on your operating system:
Windows Vista/7: C:\Users\USERNAME\AppData\Local\GOG.com\
Windows XP: C:\Documents and Settings\USERNAME\Local Settings\Application Data\GOG.com\
Then try launching the Downloader.
If that doesn't help as well, you should verify that your .NET installation isn't corrupted. To do this, use the .NET Framerowk Setup Verification Tool and choose .NET 3.5 SP1
The solution is to download without the GOG downloader.